An engine assembly generally includes an exhaust system coupled to an engine to direct an exhaust gas flow therefrom. The exhaust system may include a system body fixed to a flange that couples the exhaust system to the engine. The system body may be welded to the flange generally at an end surface thereof that generally abuts the engine after assembly. The system body and flange are typically formed from similar materials to account for the thermal expansion experienced during the welding process.
Exhaust system components such as flanges, bosses, and the like, are commonly made from cast iron because cast iron often has advantages in terms of cost, durability, packaging and NVH (noise, vibration, and harshness). Cast iron for use in exhaust systems generally includes silicon-molybdenum cast iron. Cast silicon-molybdenum iron is generally manufactured with a certain amount of graphite or some other carbonaceous materials to improve mechanical properties like hardness, strength and anti-vibration (damping) capabilities as well as some technological properties such as machinability. Cast silicon-molybdenum iron when combined with carbon often becomes weaker as the temperature increases and is subject to damage from oxidation, decarburization, and coarsening at very high temperatures. The accumulation of damage and the elevated temperature strength (the thermal strength) of the material are important factors in evaluating the durability of the exhaust component. Moreover, the ability to weld parts of the exhaust systems together is hampered by the presence of carbon (especially in the form of graphite) in cast irons.
In particular, the accumulation of surface damage together with mechanical properties at elevated temperature and resistance to thermal cyclic loading are important factors in evaluating the durability of the materials to be used in the fabrication of exhaust components.
As automotive companies increase the operation temperatures in their engines to improve efficiency and to reduce exhaust emissions, more system applications are beginning to exceed the practical working (temperature) limit of cast irons. The temperature distribution in the exhaust system is not uniform and some peak temperature areas receive more heat than other areas resulting in the formation of severe thermal stresses. As a result, materials such as cast silicon-molybdenum iron are often inadequate for use in the exhaust system. Exhaust systems therefore have to be made from other more expensive materials. This results in an increase in the cost of the exhaust system.
Some other materials like those produced by Metal Injection Molding (MIM) are not a practical alternative for fabrication of exhaust systems because they substantially increase the cost of the components related to the use of specially prepared feedstocks with fine (generally 0.5 to less than 20 microns) predominantly spherical or rounded MIM-type powders that cause great shrinkage and are linked to huge amounts of distortion in the shape of sintered products.
In addition, it is desirable for MIM products to have about 30 to about 40% of organic (hydrocarbon-based) thermoplastic binder prior to the injection molding. The presence of a thermoplastic binder leads to the formation of carbon in the product, which as detailed above is detrimental to the performance of the product.
In addition, the ability of MIM-technology to produce large-size and thicker cross-sections is limited due to difficulties in controlling debinding of complex variable shapes as well as because of contamination from carbon additives to the product. Debinding refers to the removal of resinous binders used in the process of binding the metal prior to sintering it.
As noted above, the increase in the operating temperatures of automobile engines causes non-uniform temperature distributions in exhaust components leading to excessive stresses in the component. Currently, if a material such as cast silicon-molybdenum iron is inadequate for the peak temperature, the entire system has to be made from other more expensive materials. Therefore, the manufacturing costs for exhaust systems for high temperature applications are significantly increased.